Water And Oceanography

What causes a Rogue Wave



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Although they were long thought to be nothing more than a seaman's tall tale, satellite surveillance and radar data from offshore oil platforms has now confirmed the existence of rogue waves.  Many of these waves are up to 100 feet high. One sea captain described his experience with a rogue wave as "it looked as if we were going into the White Cliffs of Dover." Even higher waves may be possible, but few ships can survive to report them.

No one knows for certain exactly what causes a rogue wave. For awhile, the impact theory was popular, but satellite footage soon found that the number of midocean meteor landings was nowhere near the number of rogue waves observed.

It is believed that rogue waves are generally caused by the same mechanism as determines the height of any other ocean wave. Ocean waves can be reinforced by other waves which interact with them, with crests adding to crests and troughs canceling out crests. The final height of an ocean wave on the open sea is the sum of all the wave heights at that spot.

Once in a while, all the crests come together just right so that there is little or no cancelation by a wave trough. This is the rogue wave. However, why that particular wave gets as high as it does is still a mystery.

It has been confirmed that a rogue wave can occur anywhere in the open sea. There doesn't have to be a nearby storm to produce unusually high waves, although it doesn't hurt. Many rogue waves are encountered close to the leading edge of storm fronts or even inside the storm itself. A stiff wind is probably a factor in generating many rogue waves.

Rogue waves are also likely to be encountered in places where normal waves, storm-tossed or otherwise, meet strong ocean currents or eddies, especially where wind action opposes the direction of the current. A common location for these kinds of rogue waves is the east side of the southern tip of Africa, where the Agulhas current meets the prevailing westerly winds and doubles back on itself. In this case, the waves caused by the Agulhas current reinforce themselves.

This region of Africa is a wave focal zone, where wave energy is focused by depth variations and strong currents. This kind of wave energy focusing can increase the frequency of occurrence of rogue waves from 3 in 10,000 waves to 3 in every thousand waves. Other focal zones can be found offshore from major river estuaries and coastal inlets.

Different types of rogue waves may have different generating mechanisms. Rogue waves have been categorized into 3 general types: walls of water, single waves, and the "Three Sisters." Single rogue waves are usually accompanied by a gigantic trough which has been described as a hole in the ocean floor. The Three Sisters are a triplet wave which are a third or so higher than the surrounding waves, and may be unique to Lake Superior. Unlike the other forms of rogue wave, walls of water seem to be capable of extended travel through the ocean without losing their structure.

Single rogue waves have been the most puzzling of all. However, a recent mathematical breakthrough may provide a partial explanation for their existence. The Periton "Soliton" is a localized solution to a complex partial differential equation known as a the nonlinear Schrodinger equation. When calculated on a distorted surface, the soliton suddenly emerges as a single extreme wave.

A rogue wave should not be confused with a tsunami. Their causes and structure are completely different. On the open ocean, even the strongest tsunamis are only a few inches high. They only grow in height when they reach shallow or otherwise enclosed water, which forces the wave energy upwards. Thus, a rogue wave cannot be caused by an undersea earthquake, because rogue waves only occur on the open sea.

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